Method of manufacturing photodiode intergrated chip

ABSTRACT

A semiconductor device manufacturing method including a process of forming a silicon oxide film by thermally oxidizing silicon in the atmosphere of oxygen gas or in the atmosphere of mixed gas of oxygen and hydrogen at a temperature of 800° C. or more in the state in which at least the silicon surface serving as a light-receiving portion of a photodiode is exposed, and a process of depositing a silicon nitride film on the silicon oxide film. At least the silicon oxide film and the silicon nitride film are finally left on the surface of the photodiode as an antireflection film.

RELATED APPLICATION DATA

This application is a divisional of U.S. patent application Ser. No. 10/181,301, filed/entered national stage Jul. 15, 2002, the entirety of which is incorporated herein by reference to the extent permitted by law. U.S. patent application Ser. No. 10/181,301 is the Section 371 National Stage of PCT/JP2001/09919, filed Nov. 13, 2001. This application claims the benefit of priority to Japanese Patent Application No. 2000-345358, filed Nov. 13, 2000.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor device including a semiconductor photo receiving region such as a photodiode and a manufacturing method thereof.

A photodiode, which serves as a photo receiving region, is generally known as an optical sensor for converting a light signal into an electric signal and is widely used as an optical sensor for control in a variety of photoelectric conversion devices.

Heretofore, on the surface of such a photodiode, an insulating film such as an SiO₂ (SiO) film has been deposited in order to protect the surface of the photodiode and also in order to suppress incident light reflected on the surface of the photodiode as much as possible.

In recent years, as a storage capacity increases and a transfer rate increases, a wavelength of laser light for use in reproducing and recording information becomes increasingly shorter in an information recording medium such as an optical disc.

However, as a wavelength of laser light becomes shorter, an absolute photosensitivity of a photo receiving region is lowered increasingly. An absolute photosensitivity obtained at about a laser light λ=400 nm in wavelength becomes approximately half of a photosensitivity of infrared rays (λ=780 nm) used in the existing optical discs.

The reason for this is that, when it is assumed that one hundred percent of light is absorbed in the silicon of the photodiode and then one hundred percent of absorbed light is photoelectrically converted into an electric signal, a photosensitivity S is expressed by the following equation (1) and this photosensitivity is proportional to a wavelength of incident light. S=qλ/hc  (1) where q represents the amount of electric charge per electron, λ represents the wavelength of the incident light, h represents the Planck constant and c represents the light velocity.

Therefore, in order to increase a photosensitivity as much as possible in response to a laser light having a shorter wavelength, it is aimed to decrease the reflectance of laser light on the surface of the light-receiving portion to zero percent, i.e., the surface of the light-receiving portion should be made substantially equivalent to the non-reflection state such that substantially one hundred percent of light can be absorbed into the silicon.

First, a minimum reflectance R (min) at a wavelength λ and a film thickness T (min) of an antireflection film used in this case are expressed by the following equations (2) and (3) respectively, where the antireflection film is made of a single layer. Note that n₁ assumes a refractive index of the antireflection film and n₂ assumes a refractive index of the silicon. Also assumed that nitrogen gas (refractive index is approximately 1) is existing between a light source and the antireflection film, R(min)=(1×n ₂ −n ₁ ²)²/(l×n ₂ +n ₁ ²)  (2) T(min)=λ/(4×n ₁)  (3)

A film having a thickness expressed by the above-mentioned equation (3) is generally called a quarter-wave film.

A study of the above-mentioned equation (2) can reveal that the value of n₁ ² is required to be close to the value of n₂ as much as possible in order to make the reflectance R(min) as small as possible.

Having compared the reflectances R (min) of the insulating materials SiO₂ and Si₃N₄ which are frequently used in a silicon-based process, for example, we have the following comparison results.

Since a refractive index of SiO₂ is 1.45, a refractive index of Si₃N₄ is 2.01 and a refractive index of silicon is 3.70 at a wavelength λ=780 nm, the results are R (min) SiO₂=(3.70−1.47²)²/(3.70+1.47²)²=7.6% and R(min) Si₃N₄=(3.70−2.01²)²/(3.70+2.01²)²=0.2%.

Subsequently, let us consider the minimum reflectances in the case of a wavelength λ=400 nm in a similar fashion

Since the refractive index of SiO₂ is 1.47, the refractive index of Si₃N₄ is 2.07 and the refractive index of silicon is 5.60 at the wavelength λ=400 nm, the results are R (min) SiO₂=(5.60−1.47²)²/(5.60+1.47²)²=19.5% and R (min) Si₃N₄=(5.60−2.07²)²/(5.60+2.07²)²=1.8%.

FIG. 4 shows the change in reflectance of the antireflection film having the single layer of a silicon oxide film (SiO₂ film) at the wavelength of 400 nm when the film thickness of the antireflection film is changed.

It is to be understood in FIG. 4 that reflectance of the antireflection film having the single layer of the silicon oxide film (SiO₂ film) is minimized to obtain the above-mentioned reflectance of 19.5% when the film thickness satisfied λ/(4×n(SiO₂))=68 nm.

FIG. 5 shows the change in reflectance of the antireflection film having the single layer of a silicon nitride film (Si₃N₄ film) at the wavelength of 400 nm when the film thickness of the antireflection film is changed.

It is to be understood in FIG. 5 that reflectance of the antireflection film having the single layer of the silicon nitride film (Si₃N₄ film) is minimized to obtain the above-mentioned reflectance of 1.8% when the film thickness satisfied λ/(4×n(Si₃N₄))=48 nm.

Specifically, it is to be understood that when the antireflection film having the single layer of the Si₃N₄ film is formed in the selected film thickness, the reflectance can be considerably lowered at any wavelengths as compared with the case in which the antireflection film has the single layer of SiO₂ film.

However, in the existing technology of silicon-based process, a method of depositing an Si₃N₄ film relies on only a CVD (chemical vapor deposition) method so that a surface level density of a silicon interface tends to increase.

In the actual photoelectric conversion by photodiode, carriers to be photoelectrically converted decrease due to a recombination of carriers on the surface.

Accordingly, when the surface level density increases as described above, a ratio at which the carriers are recombined on the surface increases so that a ratio of effective carriers to be photoelectrically converted decreases.

In particular, as the wavelength of light becomes shorter, a ratio of photoelectrical conversion in a shallow area increases so that an influence exerted due to the surface level becomes great.

Specifically, assuming that α is an absorption coefficient of light at the wavelength λ, the absorption coefficient α increases as the wavelength λ becomes short. For example, at a wavelength of 400 nm, α=7.8×10⁴/cm is satisfied. At a wavelength of 780 nm, α=1.1×10³/cm is satisfied.

Assuming that Pio is light intensity on the silicon surface, light intensity Pix in a depth x is expressed by the following equation (4): Pix=Pio·exp(−αx)  (4)

Since the absorption coefficient α increases as the wavelength λ becomes short as described above, this light intensity Pix is lowered as the wavelength λ becomes short.

Therefore, as the wavelength becomes short, the light intensity is considerably attenuated according to the depth. As a result, the ratio at which carriers are to be photoelectrically converted in the shallow area increases.

In actual practice, in the case of the antireflection film having the single layer of the Si₃N₄ film deposited by a low-pressure CVD method, it is observed that a photosensitivity in the short-wavelength range (laser wavelength λ<500 nm) suddenly declined.

In order to solve the above-mentioned problems, according to the present invention, there are provided an antireflection film capable of both decreasing a reflectance and lowering a surface level density on a photo receiving region of a semiconductor device to obtain a semiconductor device including a photo receiving region having high photosensitivity and a manufacturing method therefor.

SUMMARY OF THE INVENTION

A semiconductor device according to the present invention includes at least a semiconductor photo receiving region on a silicon substrate, at least a light-receiving area of which contains an antireflection film comprised of a laminated film having more than two layers including a first insulating film formed on the surface of the silicon substrate and a second insulating film having a refractive index different from that of the first insulating film formed on the first insulating film, and the first insulating film is comprised of a silicon oxide film which is formed by at least oxidizing silicon on the surface of the semiconductor photo receiving region.

According to the above-mentioned configuration of the semiconductor device of the present invention, since the first insulating film is comprised of a silicon oxide film formed by at least oxidizing the silicon on the surface of the semiconductor photo receiving region, the level of the silicon surface of the semiconductor photo receiving region can be lowered.

Further, since the antireflection film has the configuration such that the second insulating film whose refractive index is different from that of the first insulating film is formed on the first insulating film, it becomes possible to optimize the reflectance of the antireflection film by adjusting the film thicknesses of the first and second insulating films.

A semiconductor device manufacturing method according to the present invention includes a process of forming a silicon oxide film by thermally oxidizing silicon in the atmosphere of oxygen gas or in the atmosphere of mixed gas of oxygen and hydrogen at a temperature of 800° C. or more in the state where at least a silicon surface, which serves as a light-receiving portion of a photodiode, is exposed and a process of forming a silicon nitride film on the silicon oxide film, and the method allows at least the silicon oxide film and the silicon nitride film to be finally left on the surface of the photodiode as the antireflection film.

According to the above-mentioned manufacturing method of the present invention, since the silicon oxide film is formed by thermally oxidizing silicon in the atmosphere of oxygen gas or in the atmosphere of mixed gas of oxygen and hydrogen at a temperature of 800° C. or more in the state where the silicon surface, which serves as the light-receiving portion of the photodiode, is exposed, the level of the surface of the photodiode can be lowered by the thermal oxidation.

Further, since the above-mentioned manufacturing method of the present invention includes a process of forming the silicon nitride film on the silicon oxide film to allow at least the silicon oxide film and the silicon nitride film to be finally left on the surface of the photodiode as the antireflection film, there may be formed the antireflection film which can optimize the above-mentioned reflectance.

A semiconductor device according to the present invention includes at least a semiconductor photo receiving region for receiving light having at least a wavelength 500 nm or less on a silicon substrate, at least a light-receiving area of the semiconductor photo receiving region contains an antireflection film comprised of a laminated film having more than two layers including a first insulating film formed on the surface of the silicon substrate and a second insulating film having a refractive index different from that of the first insulating film formed on the first insulating film, and the first insulating film is comprised of a silicon oxide film which is formed by at least oxidizing silicon on the surface of the semiconductor photo receiving region.

According to the above-mentioned configuration of the semiconductor device of the present invention, since the first insulating film is comprised of the silicon oxide film formed by at least oxidizing the silicon on the surface of the semiconductor photo receiving region which receives light having a wavelength 500 nm or less, the level of the silicon surface of the semiconductor photo receiving region can be lowered.

Further, since the antireflection film has the configuration such that the second insulating film whose refractive index is different from that of the first insulating film is formed on the first insulating film, it becomes possible to optimize the reflectance of the antireflection film by adjusting the film thicknesses of the first and second insulating films.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of a main portion of a semiconductor device according to an embodiment of the present invention;

FIG. 2 is a characteristic graph showing the manner in which a reflectance of an antireflection film is changed at the wavelength of 400 nm when a film thickness of a silicon nitride film (Si₃N₄ film) in the antireflection film shown in FIG. 1 is changed;

FIG. 3 is a characteristic graph showing a relationship between each film thickness and a reflectance of an antireflection film at the wavelength of 400 nm when a film thickness of a silicon oxide film (SiO₂ film) and a film thickness of a silicon nitride film (Si₃N₄ film) are optimized in the antireflection film shown in FIG. 1;

FIG. 4 is a characteristic graph showing the manner in which a reflectance of an antireflection film is changed at the wavelength of 400 nm when a film thickness of an antireflection film having a single layer of a silicon oxide film (SiO₂ film) is changed; and

FIG. 5 is a characteristic graph showing the manner in which a reflectance of an antireflection film is changed at the wavelength of 400 nm when a film thickness of an antireflection film having a single layer of a silicon nitride film (Si₂N₄ film) is changed.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

According to the present invention, there is provided a semiconductor device including at least a semiconductor photo receiving region formed on a silicon substrate. This semiconductor device includes an antireflection film containing a laminated film having more than two layers including a first insulating film formed on the surface of the silicon substrate and a second insulating film having a refractive index different from that of the first insulating film formed on the first insulating film in at least a light-receiving area of the semiconductor photo receiving region, and in which the first insulating film is comprised of a silicon oxide film formed by at least oxidizing the silicon on the surface of the semiconductor photo receiving region.

According to the configuration of the present invention, the film thickness of the silicon oxide film is 50 nm or less in the above-described semiconductor device.

According to the configuration of the present invention, the film thickness of the silicon oxide film is 5 nm or more and 25 nm or less in the above-described semiconductor device.

Further, according to the configuration of the present invention, in the above-described semiconductor device, the first insulating film is a silicon oxide film formed by thermally oxidizing at least silicon on the surface of the semiconductor photo receiving region in the atmosphere of at least oxygen gas or in the atmosphere of mixed gas of oxygen and hydrogen at a temperature of 800° C. or more.

Furthermore, according to the present invention, the second insulating film is a silicon nitride film in the above-described semiconductor device.

According to the present invention, there is provided a semiconductor device manufacturing method including a process of forming a silicon oxide film by thermally oxidizing silicon in the atmosphere of oxygen gas or in the atmosphere of mixed gas of oxygen and hydrogen at a temperature of 800° C. or more in the state where at least the silicon surface, which serves as the light-receiving portion of the photodiode, is exposed and a process of forming a silicon nitride film on the silicon oxide film, and in which at least the silicon oxide film and the silicon nitride film are finally left on the surface of the photodiode as the antireflection film.

Further, according to the present invention, in the above-described semiconductor device manufacturing method, the silicon oxide film is formed by a low-pressure CVD method.

Furthermore, according to the present invention, in the above-described semiconductor device manufacturing method, the film thickness of the silicon oxide film is 50 nm or less.

According to the present invention, there is provided a semiconductor device including a semiconductor photo receiving region for receiving light having at least a wavelength of 500 nm or less on a substrate. This semiconductor device includes an antireflection film containing a laminated film having more than two layers including a first insulating film formed on the surface of a silicon substrate and a second insulating film having a refractive index different from that of the first insulating film formed on the first insulating film in at least the light-receiving area of the semiconductor photo receiving region, and in which the first insulating film is comprised of a silicon oxide film formed by at least oxidizing the silicon on the surface of the semiconductor photo receiving region.

According to the configuration of the present invention, the film thickness of the silicon oxide film is 50 nm or less in the above-described semiconductor device.

According to the configuration of the present invention, the film thickness of the silicon oxide film is 5 nm or more and 25 nm or less in the above-described semiconductor device.

Further, according to the present invention, in the above-described semiconductor device, the first insulating film is a silicon oxide film formed by thermally oxidizing the silicon on the surface of the semiconductor photo receiving region at least in the atmosphere of oxygen gas or in the atmosphere of mixed gas of oxygen and hydrogen at a temperature of 800° C. or more.

Furthermore, according to the configuration of the present invention, the second insulating film is a silicon nitride film in the above-described semiconductor device.

FIG. 1 shows a cross-sectional view of a main portion of a semiconductor device according to an embodiment of the present invention.

This semiconductor device is comprised of a silicon substrate 1 and a semiconductor photo receiving region comprised of a photodiode PD formed on the surface of the silicon substrate.

This photodiode PD comprises a pn photodiode having a p⁻ area 2 which serves as an anode formed on the silicon substrate 1 and an n area 5 formed near the surface of the silicon substrate 1 which serves as a cathode. Then, this photodiode PD is formed on the area isolated by an element isolating layer 4 having a LOCOS oxide film or the like near the surface of the silicon substrate 1.

This photodiode PD photoelectrically converts light incident thereon from above to provide an electric signal corresponding to the amount of incident light.

A p⁺ area 3 is formed on the silicon substrate 1 of other area isolated by the element isolating layer 4. This P⁺ area 3 is connected to the p⁻ area 2, which is the anode, to serve as an electrode lead-out area.

An interlayer insulating film 9 is formed on the surface of the silicon substrate 1. This interlayer insulating film 9 has openings at portions of the light-receiving area 10 on the photodiode PD, the end of the n area 5 and the p⁺ area 3.

Through the openings of this interlayer insulating film 9, a cathode lead-out electrode 11 is connected to the end portion of the n area 5 which is the cathode, and an anode lead-out electrode 12 is connected to the p⁺ area 3.

In this embodiment, an antireflection film 8 is formed on, in particular, the light-receiving area 10 of the photodiode PD. This antireflection film 8 is comprised of a thin SiO₂ film (SiO film), i.e., silicon oxide film 6 which are formed by thermally oxidizing silicon and an Si₃N₄ film (SiN film), i.e., silicon nitride film 7.

The silicon oxide film 6 is formed by oxidizing the silicon surface in the state where at least the silicon surface serving as the light-receiving portion of photodiode PD is exposed.

The silicon oxide film should preferably be formed by thermally oxidizing the silicon on the surface of the silicon substrate 1 in the atmosphere of oxygen gas or in the atmosphere of mixed gas of oxygen and hydrogen at a temperature of 800° C. or more, e.g., 900° C.

Since this silicon oxide film 6 is formed by oxidizing the silicon surface, it is possible to lower the level density on the silicon surface.

The film thickness of the silicon oxide film 6 formed by oxidizing the silicon surface should preferably be selected to be 50 nm or less and should be more preferably selected to be 5 nm or more and 25 nm or less.

The silicon nitride film 7 is deposited on the silicon oxide film 6 by using a low-pressure CVD (chemical vapor deposition) method or the like, after the silicon oxide film 6 has been deposited.

Since the silicon nitride film 7 deposited on the silicon oxide film 6 has the refractive index which is larger than that of the silicon oxide film 6, the silicon nitride film 7 is laminated on the silicon oxide film to efficiently decrease the reflectance as compared with the case in which the antireflection film is comprised of the single layer of the silicon oxide film.

Further, by selecting the film thickness of the silicon oxide film 6 and the film thickness of the silicon nitride film 7, it becomes possible to considerably decrease a reflectance in a certain wavelength.

Therefore, design of the film thickness of the silicon oxide film 6 and silicon nitride film 7 allows the antireflection film to be optimized by decreasing the reflectance relative to a target wavelength of light.

This point will be described below with reference to a specific example.

Let us consider the case in which the wavelength λ of incident light is equal to 400 nm as a specific example.

First, there is provided an antireflection film 8 comprising a silicon oxide film 6 having a film thickness of 10 nm and a silicon nitride film 7 on the silicon oxide film 6. FIG. 2 shows the manner in which the reflectance of the antireflection film 8 is changed when the film thickness of the silicon nitride film (Si₃N₄ film) 7 is changed at that time.

It is to be understood in FIG. 2 that the reflectance is decreased to 2.5% when the film thickness of the silicon nitride film (Si₃N₄ film) 7 is selected to be 38 nm.

It is also to be understood that a reflectance of the antireflection film 8 has a periodicity regarding the film thickness dependence.

Then, it is to be understood that a reflectance can be decreased most when the following equation (5) is satisfied with respect to a film thickness t (Si₃N₄) of the silicon nitride film (Si₃N₄ film) 7. t(Si₃N₄)=38+2N×λ/(4×n(Si₃N₄))[nm]  (5) (where N=0, 1, 2, . . . )

Thus, it is clear that an antireflection effect similar to that obtained when the film thickness of the silicon nitride film is selected to be 38 nm can be achieved by designing the silicon nitride film 7 so as to have the film thickness expressed by the equation (5).

However, in actual practice, from a film thickness control standpoint, to design the silicon nitride film having the smallest film thickness (N=0; in this case, the film thickness is 38 nm) is favorable for stabilizing reflectance most.

Next, by using the method shown in FIG. 2, a film thickness in which a reflectance can be decreased most was examined when the film thickness of the silicon nitride film 7 is changed and the film thickness of the silicon oxide film 6 was fixed to a certain film thickness in the range of 0 nm (this case is the same as the case of the silicon nitride film having single layer shown in FIG. 5) to 30 nm in which the film thickness of the silicon oxide film 6 falls.

FIG. 3 shows measured results of such examination where the horizontal axis represents the reflectance and the vertical axis represents the film thickness of the silicon oxide film 6 and the film thickness of the silicon nitride film 7.

A study of FIG. 3 reveals that the reflectance is decreased to 2.5% when the film thickness of the aforementioned silicon oxide film 6 is 10 nm and the film thickness of the silicon nitride film 7 is 38 nm.

A lower limit of a reflectance obtained in the case shown in FIG. 3 is 1.8% obtained when the silicon nitride film 7 having the single layer has a film thickness of 48 nm.

By making effective use of measured results of FIG. 3, it is possible to design the film thickness of the silicon oxide film 6 and the film thickness of the silicon nitride film 7 in response to a required reflectance.

While FIG. 3 shows measured results obtained when the wavelength λ=400 nm, the present invention is not limited thereto, and a film thickness can be designed by making a similar characteristic graph with respect to other wavelengths.

According to the above-mentioned embodiment, since the antireflection film 8 is comprised of the laminated film of the silicon oxide film 6 and the silicon nitride film 7, it is possible to decrease the reflectance as compared with the case in which the antireflection film is comprised of the silicon oxide film 6 having the single layer.

Then, by designing the film thickness of the silicon oxide film 6 and the film thickness of the silicon nitride film 7, it becomes possible to obtain the optimized antireflection film by decreasing a reflectance relative to a target wavelength of light.

Further, since the silicon oxide film 6 is deposited by oxidizing, e.g., thermally oxidizing the silicon surface, the level of the silicon surface is lowered by the oxidation.

Therefore, since a recombination of carries on the surface due to the surface-level state can be decreased, a ratio of carriers to be photoelectrically converted can be increased and hence a desired sensitivity can be maintained in the photodiode.

That is, according to this embodiment, it becomes possible to decrease the reflectance as well as to decrease the recombination of carriers on the surface.

Therefore, it becomes possible to increase a photosensitivity, in particular, in a short wavelength range.

The area in which the antireflection film 8 is formed is not always in conformity with the light-receiving area as shown in FIG. 1. The antireflection film 8 is required to be formed on at least the light-receiving area 10.

The antireflection film is not limited to the two-layer structure of the above-mentioned embodiment and the antireflection film may be comprised of a laminated film having three layers or more in which films different from the silicon oxide film 6 serving as the first insulating film and the silicon nitride film 7 serving as the second insulating film are further included. When the antireflection film is comprised of the laminated film having more than three layers, the silicon oxide film 6 of the first insulating film is laid as the lowermost layer because this silicon oxide film is deposited by thermally oxidizing the silicon.

While the present invention is applied to the semiconductor device including the single photodiode PD according to the above-mentioned embodiment, it is clear that the present invention can also be applied to a so-called photodetector IC (PDIC) including an IV amplifying circuit or the like on the same substrate, for example.

The present invention is not limited to the above-mentioned embodiment and can take various modifications without departing from the gist of the present invention.

According to the above-mentioned present invention, since it becomes possible to design the antireflection film of the photodiode in such a fashion that the reflectance may be optimized in response to the target wavelength of light, the reflectance on the surface of the photodiode can be decreased.

Further, according to the present invention, since the silicon surface is oxidized, e.g., thermally oxidized, the level of the silicon surface can be lowered.

Therefore, according to the present invention, the reflectance can be decreased and the recombination of the carriers due to the surface level on the surface of the photodiode can be decreased simultaneously, whereby the photosensitivity can be improved.

In particular, the photosensitivity is efficiently improved in the short wavelength range in which the wavelength λ satisfies an inequality of λ<500 nm wherein the photoelectric conversion is executed near the surface of the photodiode. 

What is claimed is:
 1. A process for making a semiconductor device including a semiconductor photodiode region specifically configured for receiving light with a wavelength of 500 nm or less, the process comprising: providing a silicon substrate of a first conductivity type having therein a region of a second conductivity type with a light receiving area of a semiconductor photodiode at a light receiving surface of the substrate; forming an interlayer insulating layer on the light receiving surface of the silicon substrate with an opening over the light receiving area of the semiconductor photodiode; forming an antireflection film in the opening over the light receiving area, said antireflection film including a silicon oxide film and a silicon nitride film, said antireflection film being formed by first thermally oxidizing silicon on a surface of the silicon substrate in an atmosphere of oxygen gas or in an atmosphere of mixed gas of oxygen and hydrogen at a temperature of 800° C. or more in a state in which at least said surface of the silicon substrate serving as a light-receiving portion of the region is exposed and then depositing a silicon nitride film on said silicon oxide film, wherein, at least said silicon oxide film and said silicon nitride film are left on the surface of said photodiode region in the light receiving area as said antireflection film following the process, said antireflection film overlies said entire opening; said antireflection film has a reflectance periodicity defined by its thickness, said silicon oxide film is formed to have a thickness of at most 50 nm, and said silicon nitride film is formed to have a least thickness that produces the least reflectance.
 2. The process according to claim 1, wherein said silicon nitride film is formed by a low-pressure CVD method.
 3. The process according to claim 1, wherein said silicon oxide film has a thickness in the range of 5 nm to 25 nm. 